MAME is updated the last Wednesday of every month. There are two ways of updating to the new version, 1. like you said you could just download the new full pack and delete your old one. 2. they also release a update pack every month which just contains the new/changed files, however you will need to use clrmamepro to update your existing set with the new update pack, here is a guide.
As for updating in Launchbox, once you have your roms updated, or a whole new set, just run the fullset importer again pointing it to your new rom folder, you can usually uncheck all the media downloads also as you will already have 99.9% of it.